Photographic objective.



SR I

N0. 635,472. Patented Oct. 24, |899;

C. P. GOERZ & E. VON HEGH.

PHOTUGRAPHIC UBJECTIVE.

(Application led July 2, 1898.)

CARL PAUL GOERZ AND EMIL VON HEGH, OF FRIEDENAU, GERMANY:

PHoTocRAPHlc OBJECTIVE.

SPEGIFIGATION forming part 0f Letters Patent N0. 635,472, dated4 October24, 1899.

Application filed July 2, 1898. Serial No. 685,045. (No model.)

To all whom it may concern:

Be it known that we, CARL PAUL GOERZ and EMIL voN HEGH, citizens of theKingdom of Prussia, and residents of Friedenau, near Ber.- lin, in theKingdom of Prussia and German Empire, have invented certain new anduseful Improvements in Photographic Objectives, of which the followingis a specification.

Our invention relates to a double objective for photographic purposes,as shown in the accompanying drawing. This objective consists of a pairof two-lens systems arranged in a peculiar way described hereinafter4for the purpose of attaining the following -advantages: It is aWell-known fact that the astigmatic flattening, and at the same time theneutralization, of the spherical aberration in a system of three lensesis attained by two spherical contact-surfaces of two succeeding mediums.One o f them acts as a collector and the other as a disperser for theluminous rays. Proceeding from the consideration that the efficiency ofan objective of this kind depends on the value of the difference ofrefractive indexes between the two media in juxtaposition we concludedthat further improvements, and especially a considerable increase in thesharpness of the picture, would be obtained by replacing thecontact-surfaces between two different glasses by s uch between glassand air. By this arrangement the difference between the refractiveindices will be increased six or seven times. Researches in this lineled us to the construction which, in spite of its simplicity,eliminatesin a very high degree all defects of the image produced- 'L'. e.,spherical and chromatic aberration, astigmatism, convexity of the image,coma, and distortion. In addition to this, however, we have succeeded inconsiderably increasing the general sharpness of the picture bydiminishing the soecalled intermediate error, as compared with theastigmatically-corrected cemented three-lens systems hitherto used forthe same purpose. A

Each of the two-lens systems represented in the annexed drawing consistsof a biconcave lens L' of low-refractive index and of a biconvex lens L2of a high-refractive index, as shown.

The present4 system is the nal result of 'very careful scientificresearches on the following plan: Equations were established for theelimination of the spherical aberration and for producing a constantproportion of the sines for axial rays. Dierent values were introducedfor the focal length of the first negative lens and for the distance ofthe two lenses from one another. The solution of the resulting equationof the fourth degree had as a product the respectively focal lengths ofthe second lensesand the forms for both lenses most apt for doing awaywith the two Subsequently all these forms were errors. controlled bytrigonometrically calculating a, ray near the circumference, taking intoaccount the suitably-selected thickness of glass.

This calculation showed that the calculation of the correct thickness ofthe glasses, whichv was originallyleft out of account,only slightly vinfluenced the results necessary forthe reali-1 zation of bothconditions.. Each of the coml 1 binations of lenses thus obtained wasthen examined for curvature of the image and as'- tgmatisrn byprosecuting mathematically a principal ray intersecting the axis of thesystem under an angle .of thirty-two degrees..v The astigmatic points ofthe image were thenL calculated. The comparison of the relativesituation of these points and of the plane of the image hinted at theway in which the problem was to be solved. All the systems studied inthis manner showed a considerable curvature of both astigmatic curves ofthe image. The

conditions were the most favorable ones in The result would have been aconsiderable de,-

crease in illuminating power in consequence of the great curvature ofeach lens. therefore decided not to insist on the strict It was yfulfilment of the sine conditions in the separate lens systems and onlybring the two astigmatic points of the image in the imaginaryy plane ofthe image to a coincidence, the spherical aberrationin the axis beingeach time removed by trigonometric testing. carried out in the followingway: From the objectives calculated in the mannen previ- This was" IUDously described we selected one the curvature of whose lenses stilladmitted of a suitable aperture. By varying the constant valwhen twosingle systems are exactly centrally ""ues' (radiusw'or"curvature'tmcxness of the glass, and distance of the lenses) wetried to approach our aim. This experiment succeeded completely,although the departure from the strict fulfilment of the sinecombination was nally somewhat considerable. Having found a kind ofglass which approximately possessed the calculated amount of refractionand of dispersing power for the spectrum and having made the correctionsnecessary on account of the small diderences between the refractiveindices of the lenses obtained and the values taken int-o calculation,the testing of the experimental objective in an apparatus speciallyconstructed to that purpose showed that in the single system the want ofsufficient consideration of the sine condition could not quite berecognized with precise nicety. On the other hand, the advantages of thenew system appear clearly of this kind are as follows:

Radius of curva- Length,

ture of the surin milliface. meters.

R 71.973 cl 2.246 millimeters, (thickness of the lens L.)

R2 88. 944 d 1.813 millimeters, (thickness of the layer of air betweenthe lenses.)

R3 130. 924 d3 :4.934 millimeters, (thickness of lens L2.)

The air-space between the two lenses (A) is 12.043 millimeters thick.The glass employed has the following optical properties:

Refractive index for line D of the spectrum in lens L 1.5356.

Refractive index for line D of the spectrum in lens L2 1.6112.

is equal to 50.8 for lens L lens L2.

Dispersion of the glass forming the negative lens L', n g' nn 0.01333.Dispersion of the glass forming the negative lens L2, a g' nl, 0.01325.

trate it, respectively; but these are not con-ftact-surfaces betweenglass and glass, but between glass and air-. It is obvious that for agiven aperture of a lens system the distinctness of the image aftercorrection of the spherical aberration for the peripherical rayincreases with the increase of the dierence between the refractiveindices at the contactsurfaces. The smaller these differences the deeperis the curvature of said surfaces and the more increase the intermediateerrorsthat is to say, the spherical deviation of the rays which passbetween the center and the periphery. These differences of refractiveindices consequently in the three-lens system amount to: On the surfaceZ, 0.06; on the surface S, 0.10. In the case of the two-lens system, onthe contrary, on the surface Z, 0.53; on the surface S, 0.61,consequently nine or six times more, respectively, and therefore thesurfaces themselves will possess a 4considerable flattening ascomparedwith those of t the three-lens anastigmatics. The verycousiderably increased clearness of the image is fully explained by thiscircumstance. The new two-lens system can therefore be regarded as asystem derived from the three-lens system by decreasing the power ofrefraction of the inclosed lens until it becomes equal to 4therefraction of the air-i. e., equal to 1 and that it is no longer a glasslens, but an air-lens inclosed between `a loiconcave lens and a biconvexlens. The condition sine qua non for getting to the above-stated objectspherical, astigmatic, and chromatic corrections with increase in theclearness of the image-is formed by the following` criteria. The backsurface of the rst biconcave lens of low-refractive power has to receivea deeper curvature than the front surface of the biconvex lens of highrefraction separated from' it by a layer of air. first surface of thenegative lens, just as in the case of the above-mentioned three-lenssystem, is curved flatter than the back surface of the biconvex lens. Ifthis condition were not fulfilled, the elimination' of the astigmatism,as well as of the spherical aberration, would prove impossible. Theairspace left between the biconcave and the biconvex lensl has the formof a convex-concave meniscus of positive focal distance. If, forinstance, the achromatism of the two systems of lenses can only beapproximately attained with the available species of glass, we are ableto cancel the. still remaining error without any difficulty by composingone ef the lenses of two lenses of approximately equal refrac- On theother hand, the

Idd

Ird-

tion, but of different dispersion cemented to-. y

gether.

Having now particularly described and ascertained the nature of our saidinvention and in what manner the same is to be performed, we declarethat what we claim is- IssA A double objective for poses consisting intwo two of which is composed of a low-refractive power (refr 5 toca.1.53) and abiconve frac'tve power (refractive 1.61) both having nearlying power and leaving space having the form of a ro the outside surfaceof the photographic pur- -lens systems, each biconcave lens of activeindex equal x lens of high-reindex equal to ce. the same dispersbetweenthem an airpositive meniscus; negative lens having a longer radius thanthe outside surface n

